The fast, efficient and error-free transmission of digital information from one point to another has become increasingly important. Many communications systems exist which permit digital information to be transmitted over various types of communication channels, such as wireless channels, fiber-optic channels, and wire line channels.
The present invention will be described in the context of a wire line communications channel, such as a telephone line, which utilizes a twisted pair of copper wires. It is noted that the use of the present invention is not limited to wire line systems, as those skilled in the art will appreciate from the discussion hereinbelow.
A modem is typically used to transmit and receive digital data over a telephone line. Modems employ a modulator to transmit the digital data over the telephone line and a demodulator to receive digital data from the telephone line. One common modulation technique is known as digital multi-tone (DMT) modulation, which requires a discrete multi-tone transmitter and a discrete multi-tone receiver at each modem in a communication system. Often, those skilled in the art refer to such modems as employing a DMT physical layer modulation technique. The systems disclosed in the '478 and '570 applications (as well as the present disclosure) rely on receiver-end windowing to mitigate the problem of spectral leakage in the standardized DMT signaling format. In those systems, windowing is applied only at the receiver side, in order to retain compliance with the present industry standard. The windowing function generally improves performance in the presence of spectrally localized RFI, colored noise and/or insufficient TEQ equalization.
However, this improvement is generally at the expense of reduced performance for DMT bins (sub-channels) in the frequency range where additive white Gaussian noise (AWGN) is the dominant impairment. Therefore, in the '478 application, a combination of windowed and non-windowed bin processing was utilized, selectively retaining frequency-domain windowing only on bins where it is determined that the SNR performance is improved. The side effect of windowing is introduction of inter-bin interference, so that some form of decoding of the partial response signaling (PRS) type is needed. In the '478 application, TEQ channel equalization and DFE (decision feedback equalizer) type decoding is utilized, which is a good compromise between performance and complexity of practical realization.
Decoding based on maximum likelihood sequence estimation (MLSE) has the potential to give better performance than DFE decoding, but at the expense of higher implementation complexity, particularly when it is used in a system like that described in the '478 application. However, with an equalization technique as disclosed in the '570 application, substantially reduced MLSE complexity is achieved, primarily by using the well-known Viterbi algorithm.
Various systems and methods of additional background interest are described in the following references, which will be referred to further below, referenced by figures in brackets, and which are expressly incorporated by reference herein.    [1] R. Li and G. Stette, “Time-Limited Orthogonal multicarrier modulation schemes:” IEEE Tr. On Communications, February/March/April 1995    [2] D. Dardari, “MCM system with waveform shaping in multi-user environments: Effects of fading, interference and timing errors”, Globecom, 1997    [3] H. Kobayashi, “Correlative Level Coding and Maximum-Likelihood Decoding”, IEEE Transactions on Information Theory, September 1971    [4] P. P. Kabal and S. Pasupathy, “Partial-Response Signaling”, IEEE Tr. On Comm., September 1975    [5] S. B. Slimane, “Performance of OFDM systems with time-limited waveforms over multipath radio channels”, Globecom, 8–12 November 1998.
Of particular interest is reference [5], where MLSE was applied on the receiver side in the transmitter-end windowed multicarrier (MC) system of [2], but without skipping every second subchannel. However, transmitter-side windowing is non-compliant with current standards. Further, although transmitter-side MLSE is used in the specific arrangement disclosed in [5], that arrangement fails to provide any suggestion as to how MLSE, windowing or noise cancellation could be used on the receiver side.